A variety of photographic imaging systems are known for producing dye images wherein the mobility of the dye or its precursor is controlled in an imagewise manner by addition to ethylenic (vinyl) unsaturation sites in an associated hardenable organic component. The hardenable component undergoes photoinduced addition reactions, typically polymerization and/or crosslinking, at the ethylenic unsaturation sites which hardens the organic component and thereby imparts its ability to modulate mobility of the associated dye or dye precursor.
A simple illustration of such an imaging system is a negative working photoresist which contains an imaging dye. Imagewise exposure of the photoresist followed by development leaves a dye image in exposed areas. While there are a plethora of known negative working photoresists which might be cited, Tan et al U.S. Pat. No. 4,289,842 illustrates photoresists employing crosslinkable copolymers containing ethylenic unsaturation in pendant groups, Lindley U.S. Pat. No. 4,590,147 illustrates photoresists including vinyl oligomers, and Fuerniss U.S. Pat. No. 4,497,889 illustrates photoresists containing vinyl monomers.
Illustrative of more elaborate imaging systems capable of producing transferred dye images are Sanders et al U.S. Pat. Nos. 4,399,209 and 4,440,846. A dye precursor and a hardenable organic component containing ethylenic unsaturation sites are coated together on a support in rupturable microcapsules. Imagewise exposure to actinic radiation hardens the organic component by inducing addition at its ethylenic unsaturation sites. Upon subsequent rupture of the microcapsules only the dye precursor in unexposed microcapsules have the mobility to transfer to a receiving sheet where a viewable dye image is formed.
Since the hardenable organic components containing ethylenic unsaturation exhibit only limited direct response to exposing radiation, it is common practice to include an initiator of the ethylenic addition reaction. In practice negative working photoresists are typically imagewise exposed using radiation wavelengths in the near ultraviolet region of the spectrum. While Sanders et al recognize that imaging exposures are possible with various forms and wavelengths of radiation, the preferred wavelengths of exposure are limited to the ultraviolet and the blue, up to about 480 nm, with the initiators for the ethylenic addition reaction disclosed being those which are responsive at these wavelengths of exposure.
In order to achieve higher levels of sensitivity (i.e., higher imaging speeds) than can be achieved with a single initiator, it is common practice in preparing imaging compositions to employ coinitiators. One of the coinitiators is a photosensitizer. Photosensitizers are relied upon to capture photons of exposing radiation. The remaining coinitiator is referred to as an activator. The activator is not relied upon to respond directly to exposing radiation, but rather adjacent activator and photosensitizer molecules react, following excitation of the latter by photon capture, causing release of a free radical which in turn induces immobilizing addition reactions at sites of ethylenic unsaturation.
Imaging systems which rely on a combination of an activator and a photosensitizer are typically exposed in the ultraviolet or blue portion of the spectrum. The necessity of using shorter imaging wavelengths places constraints on the master or pattern employed for imaging. For example, a master which presents a longer wavelength visible dye image, but exhibits little variance in ultraviolet or blue transmittance is not well suited for imagewise exposing an imaging system which responds only to ultraviolet or blue radiation. Further, such imaging systems are disadvantageous and have not found acceptance in producing multicolor images.
There are several difficulties in attempting to formulate imaging systems which respond to wavelengths longer than 550 nm. For example, the energy contained in light photons declines as the wavelength of the radiation increases. This places increased demands on any photosensitizer coinitiator intended to absorb in longer wavelength regions of the spectrum. Additionally, generally more elaborate molecules are required to absorb longer wavelength radiation. This increases the bulk of the photosensitizer. Related to this, reduced thermal and storage stability have been a point of concern.
It is generally accepted that photosensitizer coinitiators function by photon absorption to lift an electron from an occupied molecular orbital to a higher energy, unoccupied orbital. The spin of the electron lifted to the higher energy orbital corresponds to that which it exhibited in its original orbital or ground state. Thus, the photosensitizer in its initially formed excited state is in a singlet excited state. The duration of the singlet excited state is limited, typically less than a few nanoseconds. The excited photosensitizer can return from its singlet excited state directly to its original ground state, dissipating the captured photon energy. Alternatively, the singlet excited state photosensitizer in some instances undergoes intersystem crossing through spin inversion to another excited state, referred to as a triplet state, wherein lifetimes are typically in the microsecond to millisecond range. Since photosensitizer coinitiators which exhibit triplet states remain in an excited state for time periods that are orders of magnitude greater than photosensitizer coinitiators which exhibit only singlet excited states, a much longer time period is available for reaction with the paired activator coinitiator.
Specht and Farid U.K. No. 2,083,832A discloses photopolymerization coinitiators including azinium activators and amino-substituted photosensitizer (e.g., amino-substituted ketocoumarin) coinitiators which exhibit triplet states on excitation. An essentially cumulative disclosure is provided by Research Disclosure, Vol. 200, December 1980, Item 20036. Research Disclosure is published by Kenneth Mason Publications, Ltd., Emsworth, Hampshire P010 7DD, England. As illustrated by Specht, Martic, and Farid, "A New Class of Triplet Sensitizers", Tetrahedron, Vol. 38, pp. 1203-1211, 1982, these amino-substituted 3-ketocoumarins exhibit intersystem crossing efficiencies ranging well above 10 percent--e.g., from 0.18 to 0.92 or 18 to 92 percent, measured in polymer.
Heseltine et al and Jenkins et al U.S. Pat. Nos. Re. 27,922 and Re. 27,925 disclose N-oxyazinium initiators for photocrosslinking and photopolymerizaation, respectively, which are capable of responding directly to visible light. The difficulty of having the free radical generating azinium initiator capable of directly absorbing light is that synthesis is relatively complex. Further, synthesis and handling of the initiator must be performed in the absence of light, which is a significant disadvantage.
In concurrently filed, commonly assigned patent applications, Ser. Nos. 933,658 and 933,660, titled NEGATIVE WORKING PHOTORESISTS RESPONSIVE TO SHORTER WAVELENGTHS AND NOVEL COATED ARTICLES and NEGATIVE WORKING PHOTORESISTS RESPONSIVE TO LONGER WAVELENGTHS AND NOVEL COATED ARTICLES, respectively, negative working photoresists are disclosed comprised of a film forming component containing ethylenic unsaturation and capable of selective immobilization as a function of ethylenic addition, and activator and photosensitizer coinitiators for ethylenic addition. The activator is an azinium salt, and the photosensitizer is a dye having a reduction potential which in relation to that of the azinium salt activator is at most 0.1 volt more positive.
In concurrently filed, commonly assigned patent application, Ser. No. 933,657, titled ENHANCED IMAGING COMPOSITION CONTAINING AN AZINIUM ACTIVATOR, an imaging composition is disclosed comprised of an organic component containing ethylenic unsaturation sites and capable of selective hardening by addition at the sites of ethylenic unsaturation, an azinium salt activator, a photosensitizer having a reduction potential which in relation to the reduction potential of the azinium salt activator is at most 0.1 volt more positive, and an image enhancing amount of benzene substituted with an electron donating amino group and one or more groups capable of imparting a net Hammett sigma value electron withdrawing characteristic of at least +0.20 volt to said benzene ring.